1. Field of the Invention
The present invention generally relates to a disk driving apparatus on which a brushless motor for driving a recording/reading disk is mounted.
2. Description of Related Art
In recent years, as a novel recording/reproducing method adapted to higher recording capacity of a blue-ray disc and an HD-DVD is employed, the demand for higher rotation accuracy of a recording/reading disk used for the method rises. In particular, to prevent erroneous operation in recording/reading and suppress rotation vibration of a recording/reading disk, there is a tendency that a mounting part of the brushless motor on which a recording/reading disk is to be mounted is requested to have run-out preventing accuracy which is about twice as high as that of conventional one. When a picture or design is drawn on a surface on the back of the recording surface of a recording/reading disk, the disk mounted on the mounting part may be rotated at low speed. It demands a brushless motor having the high rotation accuracy even when a recording/reading disk is rotated at low speed, particularly, at 100 rpm or lower.
Simultaneously, reduction in price of the brushless motor is demanded. Consequently, a slide bearing such as an oil-impregnated sintered bearing is recently being used as the bearing. When the slide bearing is used, however, a so-called half-speed whirl phenomenon that a frequency component which is the half of rotation speed conspicuously appears occurs. The phenomenon causes vibrations and irregular rotation; therefore hinders improvement in rotation. To prevent occurrence of the phenomenon, countermeasures are taken to prevent whirl of a rotary shaft by tilting the rotary shaft in one direction and supporting it with a bearing by a method of narrowing the gap between the rotary shaft and the bearing or a bias magnetic preloading method of applying a magnetic bias on one side.
In the method of narrowing the gap between the rotary shaft and the bearing, however, the highly-accurate tolerance are requested for the outside diameter of the rotary shaft and the inside diameter of the bearing, and the production cost increases dramatically. Further, when the rotary shaft and the bearing are inserted at the time of assembly, they come into contact with each other due to the narrow gap, and the slide bearing may be damaged. In addition, the bearing is cut by the contact between the rotation shaft and the bearing during rotation, and the gap may be widened with time.
In the method of applying the bias magnetic preload, when the bias magnetic preload direction is opposite to the gravity direction, the bias magnetic preload and the gravity cancel out each other, so that there is no point of applying the bias magnetic preload. Further, when the direction of the bias magnetic preload and that of the gravity are the same, an excessive force acts in one direction of the shaft, and the bearing is excessively cut. As a result, an influence may be exerted on the bearing life.